In general, a four-cycle four-cylinder engine may be mounted upon a large sized motorcycle and, in such case, it will be particularly required for the engine to be of the high-speed rotation and high output type in view of the maneuverability of the motorcycle. Exhaust gas discharged from the engine is introduced, through means of a plurality of exhaust pipes, four in the present case, into an exhaust mufflers serving to silence the exhaust noise and the exhaust gas is then exhausted. However, since the longitudinal length of the body of the motorcycle is itself limited to a predetermined range, the lengths of the exhaust pipes and the muffler to be assembled upon the motorcycle body are themselves limited in connection with the length of the motorcycle body. This involves a problem such that the sizes and the lengths of the exhaust pipes and the muffler cannot be designed only upon the bases of the performances thereof because of the inevitable limits of the size and the length of the motorcycle body. Therefore, with an engine of the high-speed rotation and high output tYpe, there is a case where the engine has which a rotation period in which the output is partially adversely dropped or reduced and, hence, the motorcycle is operated with less maneuverability.
In order to obviate such defect, in accordance with conventional technology, exhaust chambers are provided for two of the four exhaust pipes, the other two exhaust pipes being connected through means of a communication pipe, or alternatively, a 180.degree. ignition type separator, described later, is disposed so as to reduce the decrease of the output. When the 180.degree. ignition type separator is utilized, it is required to use exhaust pipes each having a relatively short length and, hence, the pulsation of the exhaust gas cannot be effectively utilized. In addition, a collecting cylinder must be positioned upon the front side, resulting in the limitation of the mounting characteristics of the motorcycle body.